Taran drawing Drynwyn in the final book, and slaying the Nigh Invulnerable Cauldron-Born in a single blow.
Also, when Taran fights Dorath, he uses a sword he forged himself, while Dorath fights with the sword he stole from Taran (which was given to Taran at the beginning of The Black Cauldron). Taran's new blade proves so much better that it actually shatters the old one.
This is a symbolic way of showing that Taran's character has been developing since he obtained the sword to begin with, with the now older, more Taran overcoming his younger, inexperienced self.
Dallben gets one when Pryderi comes with a group of soldiers to kill him. For four books he's been called 'the greatest sorcerer in Prydain' but done little more than a little light divination between naps. Here his power is finally shown.
Rhun, previously identified as a well-meaning clod, pulls off a beautiful one in The High King when he confuses the enemy into thinking he's brought a giant army with him.
Prince Ellidyr repents of his pride, bullying and treachery - but knows they are innate to his character and that his repentance won't last. He solves the 'insoluble' problem of how to destroy the Black Cauldron via Death Equals Redemption.